In an era of aging infrastructure, increasing environmental degradation, and fierce international competition for market share, nondestructive testing has become a financial and social necessity offering potentially enormous benefits. It is one of the primary vehicles companies rely on in their pursuit of ever higher product quality. Ultrasonic inspection based on very high frequency sound propagation in structural materials is probably the most widely used nondestructive testing method to detect lack of bond, porosity, internal cracking, corrosion, and other structural defects in welded and brazed joints.
The outstanding sensitivity of ultrasonic inspection for detecting material discontinuities is due to the large difference in elastic modulus between the structural material to be inspected and the air usually filling the discontinuities to be detected. The relative contrast between the elastic moduli of the reflecting layer C.sub.1 and the surrounding host material C can be defined as follows.
.xi.=.pi.C/C.sub.1. PA1 R=.xi.d/.lambda.
For example, acoustic contrast is as high as 6.7.times.10.sup.6 for an air-filled crack in steel, but only 3.5.times.10.sup.2 for a water-filled crack. For thin reflective layers, the reflection coefficient can be approximated as:
where d denotes the thickness of the layer and .lambda. is the ultrasonic wavelength in the host material. For example, the acoustic wavelength in steel is approximately 2.2 mm at 2.5 MHz. Assuming a modest 10% threshold sensitivity, an extremely thin 0.3-.ANG.-wide air gap or a somewhat wider but still impressive 0.6-micron-thick water-filled crack can be detected at this frequency.
This remarkable sensitivity to material discontinuities represents also the most severe limitation for ultrasonic nondestructive testing because of the requirement that good acoustic coupling be maintained between the transducer and the object to be inspected (the specimen) without the slightest discontinuity between them. This goal can be achieved on flat, smooth surfaces by using some kind of fluid coupling to fill the inevitable thin, typically sub-micron, gap between the transducer and the specimen. The same technique is much less effective on curved surfaces, however, where the width of the gap between the transducer and the specimen is inherently wider. The simplest curved surfaces are convex and concave cylindrical surfaces that can be relatively easily coupled by matching cylindrical wedges. FIGS. 1A-1D illustrates how special contoured transducer wedges polished to the exact radius of the specimens to be inspected can be used in axial or circumferential directions. Although such custom-made transducer wedges can improve coupling on curved surfaces by eliminating rocking and couplant build-up underneath the wedge, precise matching is often prevented by inevitable variations in the surface curvature of the specimens. This variation becomes especially crucial when large-aperture, e.g., multiple-element array, transducers are used in electronically scanned multi-channel systems.
Contoured transducer wedges also limit the operator's ability to freely align the transducer. On a flat surface, an angle-beam transducer can be translated in both the forward direction and sideways as well as rotated in the plane of the surface. As is readily apparent from FIGS. 1A-1D, contoured angle-beam transducers with cylindrical faces can also be translated both forward and sideways, but cannot be rotated as the radius of curvature is finite in one direction and infinite in the other.
The coupling problem is further exacerbated on double-curved surfaces. For example, thick plates and rods bent in one direction at a principal convex radius of curvature inherently assume a secondary concave curvature in the orthogonal direction due to the so-called Poisson effect as it is shown in FIG. 2. Such surfaces are very difficult to accurately match with precision contoured angle beam transducers of double-cylindrical faces, and the transducer cannot be laterally translated or rotated at all without losing contact between the wedge and the specimen. The inevitably larger surface gap requires the use of large amounts of high-viscosity couplants, but even this undesirable solution is rendered useless when the radius of curvature exhibits a significant variation from specimen to specimen.
In generator field coils of dynamoelectric machines, high quality braze joints are of paramount importance to maintain generator quality and integrity. The usual visual inspections method do not address the problem of hidden defects caused by operator error, improper cleanness, contamination of the joint area by oil, grease, varnish, etc. all prevalent in a heavy industrial environment. Other nondestructive testing methods, because of the very large size and weight of the fields, preclude their use. The double-curvature (discussed above) of the field coils also limits the standard single transducer ultrasonic method because it is time consuming, expensive and not suitable for a production environment.
One conventional technique for providing acoustic coupling on irregular, curved surfaces is to equip the transducer with an oil-filled rubber balloon or wheel that can easily conform to the exact shape of the specimen. However, in tough industrial environment the necessarily very thin rubber balloon often ruptures, spilling the oil couplant. This presents an unacceptable contamination risk in certain applications such as brazing and generator field winding operations, where any contamination, however minor, is totally unacceptable.
A solid rubber coupling would be more desirable in the applications discussed above. However, special low-attenuation rubbers have inherently high stiffness requiring relatively thick rubber cushions to conform to the irregular surface, and therefore the transmission loss is still significant. On the other hand, special low-stiffness rubbers are usually excellent sound absorbents therefore cannot be used as padding materials for ultrasonic coupling purposes. Thus, solid rubber coupling techniques improve sensitivity but are not completely satisfactory.